This invention relates to apparatus for positioning a transducer relative to a recording medium. It relates more specifically to an intermittent feedng device that moves a transducer between the tracks of a rotary recording medium such as a magnetic disk.
There are numerous different types of transducer positioning devices. The general type with which we are concerned here moves the transducer by means of a belt loop that is advanced in one direction or the other by a stepping motor-driven capstan engaged in the loop. Examples of such belt-driven positioning devices are contained in U.S. Pat. Nos. 3,749,828 and 3,946,439.
In general, these conventional devices employ an endless belt loop that is stretched between a capstan and a tensioned idler. A carriage supporting the transducer is connected to the belt at a location between the capstan and idler so that when the belt is moved in one direction or another, the carriage is moved one way or the other along a fixed track relative to the recording medium. Of course, the belt cannot be permitted to wind up on the capstan so that belt segments overlap because this would change the effective radius of the capstan. Then, for each angular increment of the capstan, the transducer would be moved a distance that would vary depending upon the amount of wrap on the capstan. Such would result in uneven spacing between the tracks on the recording medium which is the reason why a single belt loop is generally used in positioning devices of this type. The belt is advanced in increments by means of a stepping motor which, when pulsed, rotates the capstan through very small angular distances or steps. Typically the motor shaft may have as many as 200 steps per revolution.
Also in devices of this type, the belt is invariably secured at a point on the periphery of the capstan to prevent relative movement between the belt and the capstan in order to insure that each step of the stepping motor advances the transducer precisely the same distance along its track. Because of this connection, neither the shaft of the stepping motor nor the capstan can be rotated in either direction through one revolution. For as the capstan rotates, different incremental belt lengths leave the capstan at one side thereof, while others arrive at the capstan at the opposite side thereof. Most often the connection point between the belt and the capstan is located at the end of the loop when the transducer carriage is situated midway between the capstan and the idler. This permits the capstan to be rotated in either direction through an angle that is theoretically 90 degrees before the connection point to the capstan is rotated around to where the belt departs from the capstan.
In actual practice, however, the angle is much less, being only on the order of 75 degrees because of stresses developed in the tape as the connection point approaches the departure point. In other words, the total angular excursion of the capstan is only about 150 degrees. This means that a 200 step per revolution stepping motor can rotate the capstan through 150 degrees or 77 steps. Therefore, the transducer can only be incremented 77 steps radially across the recording medium. Since each of these steps defines a track on the recording medium, this severely limits the track density on the medium.
It is possible to operate some stepping motors in a half-step mode thereby doubling the track density, not, however, without paying a penalty in terms of performance and transducer positional accuracy. Also in some cases because of the small increments involved, a relatively large diameter capstan is required which has appreciable inertia. Therefore the response of such a device is rather slow so that a relatively long time is required to move the transducer between tracks on the associated recording medium.